Alternative illumination system for extreme ultraviolet mask inspection based on Fourier synthesis technology.

This study proposes an alternative illuminator based on the Fourier synthesis technology that provides a powerful and flexible way of controlling the coherent properties of illumination for extreme ultraviolet mask inspection. The illuminator achieves coherence control by programming the incident beam scanning a Fresnel zone plate and thus can provide free pupil-fill patterns. In this work, a visible laser-based laboratory microscopic imaging platform has been developed using the illuminator. The spatial resolution and the dense-line image contrast were experimentally evaluated for various coherence factors when a disk and a dipole pupil-fill pattern were applied, respectively. The results are in good agreement with the theoretical calculation of the Rayleigh criterion and the contrast transfer function, which validates the proposed new illuminator. The reliable laser-based imaging platform sheds light on designing and improving EUV mask inspection systems based on synchrotron radiation light sources. The proposed new alternative illuminator will be used in an EUV microscopy at the Shanghai Synchrotron Radiation Facility in future work.